This invention relates to the measurement of surface properties of materials, and, more particularly, to the ellipsometric measurement of surface conditions and films on surfaces.
The quality and condition of its surfaces often determine the suitability of a material for use in an industrial process. To cite some examples, if a material is to be painted or bonded to another material, the presence, type, and amount of contaminants such as grease or dirt often determine the success of the painting or bonding operation. Coatings or oxides are placed onto the surface of semiconductor wafers during the fabrication of microelectronic components, and the thickness of the coating or oxide must be established within critical limits for subsequent operations to be successful. The cleanliness of memory disk drive heads must be checked periodically to be certain that contamination does not interfere with read/write functions. These examples are illustrative and not limiting, as there are literally thousands of examples where surface properties are determinative of the operability of the material in its intended use.
In recognition of the critical importance of surfaces, a number of techniques have been developed to observe and measure surface characteristics. Surfaces may be viewed either directly or with magnification. They may be measured by devices such as profilometers which give a quantitative measurement of roughness. In the last 20 years, sophisticated techniques such as Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), Scanning Electron Microscopy (SEM), and standard Ellipsometry have been employed for characterizing surfaces, almost always in a laboratory environment.
However, when considered for use in an industrial environment, all of these techniques have serious drawbacks. Most of the available instruments are bulky and expensive to purchase and maintain in operation and calibration. In many cases, such as LEED, AES, and SEM, the pieces under study must be placed into a high-vacuum chamber, thereby limiting the size of the pieces and the rates at which they may be examined.
The greatest drawback, however, is that most of these techniques are not readily adapted to acceptance testing and real-time testing on an industrial scale by relatively unskilled personnel or under computer control. For example, standard ellipsometry may be used to determine the thickness of an oxide coating formed on a surface of a semiconductor material by a particular heat treatment as heat treatment control parameters are being developed. This technique typically is not practical to check every piece subsequently given the nominally same heat treatment.
Instead, it must be assumed that uncontrolled variables do not enter the process and that each heat treatment is successful in producing the desired surface. This assumption fails where there is some error or malfunction in the production operation, and many pieces may be given a bad treatment before the problem is discovered. The most sophisticated inspection techniques therefore cannot be used for real-time control of a production operation, where measurements of all or a large portion of completed parts are used to check whether the production operation is proceeding properly, or whether some adjustment is required.
Additionally, when utilizing many highly advanced surface characterization techniques it is difficult to know whether the characteristic measured is really important and determinative of acceptability of the surface. That is, measurements may be made of surface characteristics, but one is then faced with the problem of deciding whether that measurement is at all relevant to the planned usage of the material and, if so, what the limits of acceptable variation of the measured characteristic might be in order to ensure success of the subsequent processing.
There has therefore existed a continuing need for various types of surface monitoring apparatus which are sensitive to microscopic surface characteristics, are inexpensive, portable, and compact, do not require that the surface to be studied be placed into a vacuum chamber, and allow the development of acceptability and quality control tests which may be utilized by relatively unskilled personnel or under computer control for testing large surfaces and numbers of parts. In particular, it would be useful to have a surface inspection instrument that can be placed near to, or scanned over, the inspection surface itself. In many existing situations, small samples must be taken from the surface and placed into the instrument for analysis, under the implicit, but often erroneous, assumption that the small sample is truly representative of the entire surface.
Two important responses to this need for sophisticated surface inspection techniques usable in a production environment are disclosed in U.S. Pat. Nos. 4,381,151 and 4,590,376, which describe apparatus and techniques for performing light reflection and secondary photoelectron inspections of surfaces, respectively, that may be used in quality control and related activities. The approach of the '376 patent has achieved considerable commercial success in this regard. Each of these approaches has its limitations, however, and other approaches are needed.
However, the need for even further improved apparatus for performing inspections and analysis in a production environment remains. The present invention fulfills this need, and further provides related advantages.